Metastasis is the leading cause of cancer death. Once cancer has spread to and beyond regional lymph nodes, complete eradication of the tumor by surgical intervention is nearly impossible (S. A. Rosenberg, Cancer: Principles and Practice of Oncology, V. T. Devita, S. Hellman and S. A. Rosenberg, Eds. (Lippincott-Raven, Philadelphia, 1997), p. 295). Instead, the whole body must be treated with radiation or chemotherapy, which are notoriously toxic to normal cells and tissues (V. T. Devita, Cancer: Principles and Practice of Oncology, V. T. Devita, S. Hellman and S. A. Rosenberg, Eds. (Lippincott-Raven, Philadelphia, 1997), p. 333). These facts underscore the need to identify the fundamental causes of metastasis and to translate this information into more effective and less toxic therapies. In spite of recent advances in studies of the basic mechanisms of metastasis, relatively little of this progress has translated into therapeutic approaches that minimize damage to normal tissues.